Diesel engine exhaust gases contain a number of noxious gases, such a nitrogen oxides, sulphur oxides and carbon oxides, as well as un-burnt hydrocarbons, carbon and other particles. The amount of sulphur oxides in the exhaust gases is dependent primarily upon the sulphur in the fuel and is controlled by the quality of the initial crude oil and the refining techniques used in the preparation of the fuel. However, the other materials can be treated so as to render them less obnoxious.
It is therefore common practice to pass the exhaust gases through a silencer assembly which contains one or more treatment chambers containing a catalytic converter in which the lower nitrogen oxides are converted to NO2. The carbon particles and droplets of un-burnt hydrocarbons, for convenience collectively denoted hereinafter as particulates, are removed from the exhaust gas stream by a metal gauze or mesh or a ceramic filter element. The NO2 and oxygen in the gas stream react with the particulates trapped in the filter element to form carbon dioxide and water, which are then discharged with the other exhaust gases.
If desired, the filtered gases can be subjected to reduction of remaining nitrogen oxides to nitrogen by injecting urea into the gas stream after it leaves the filter element but before it leaves the silencer assembly. The treated gas stream is then passed over an oxidising catalyst to convert residual ammonia from the urea to nitrogen and water, which are acceptable exhaust emissions. The net result is a typical reduction in noxious components of the exhaust gases of over 90%.
Silencer assemblies containing such treatment chambers are typically built on a modular basis as a series of generally cylindrical or oval cross section module units, each containing an element, the treatment element, required to achieved one of the desired treatments on the exhaust gas stream. Typically, each module comprises the treatment element as a cylindrical body surrounded by a shock absorbent ceramic sleeve, located by internal circumferential annular ribs or flanges within a metal, usually stainless steel, tubular body member. Thus, one module may contain the catalyst carried on a foraminous ceramic or other support, for example a rolled corrugated metal sheet with an interleaf of a flat metal sheet between each layer of the corrugated sheet to form a plurality of triangular or other cross section axial passages in a cylindrical or annular support, or as a noble metal wire mesh through which the exhaust gases pass. A second module may contain the cast porous or fritted ceramic filter which traps the particulate material and upon which those particles burn in the presence of the NO2 formed within the catalyst module. If desired, the catalyst support can also serve part or all of the function of filtering the gas stream. Other modules can be used for other treatments, for example the injection of urea. Additionally, the modules will also serve to attenuate the noise emitted by the engine which the modules serve.
In addition to allowing the designer greater flexibility in designing the treatment chambers to achieve optimum treatment of the exhaust gasses, the use of a modular construction enables the operator to remove one or more of the modules for cleaning and servicing or replacement. For example, it has been found that the performance of the filter module can be prolonged if the module is removed and replaced in the silencer assembly with its orientation reversed, thus reversing the direction of flow of the exhaust gases through the filter element.
Such modules are preferably secured together as an axial series of components by releasable joint mechanisms in which outwardly extending radial flanges at opposed ends of adjacent modules are secured together with a compressible gasket forming a gas-tight seal between the opposed faces of the flanges. The accepted method for securing the flanges together, whilst retaining ease of dismantling the joint, is to form the flanges as opposed axially inclined radially outwardly directed shoulders and to apply a V section strap with a circumferentially acting clamp upon the shoulders of two adjacent modules. Upon tightening of the clamps, the straps contract radially upon the taper of the shoulders and thus lock the modules together and clamp the shoulders axially upon a gasket located between them to form a gas-tight joint between the modules. Silencers incorporating treatment chambers using such a modular construction are commercially available from the Applicants under the Trade Mark Greencat.
For convenience, the term silencer will be used hereinafter to denote in general chambers for the treatment of exhaust gases to remove noxious materials from the gas stream and which also by design or co-incidentally may attenuate the exhaust noise from an internal combustion engine.
However, such a construction using a plurality of modules may result in a silencer assembly which is excessively long. On the other hand, problems are encountered where it is desired to nest modules one within another so as to reduce the axial length of the silencer. It is necessary completely to dismantle such a nested assembly in order to gain access to a module contained within another, so as to be able to remove the circumferential straps securing that module to its neighbour, which is time consuming.
We have devised a form of assembly for a silencer which provides a cheaper and more compact assembly, yet which retains the modular construction and ease of manufacture and flexibility of assembly of the modules.